It begins with a dry crack, a musket shot from some heavenly picket line, then expands into a cannonade—boom, boom, boom!—loud enough to rattle teeth. The sky above the Gulf of Policastro, on the coast of southern Italy about a hundred miles below Naples, is clear, but the Serra del Tuono, “the mountain range of thunder,” is living up to its name on a winter day. The noise is a portent, and sure enough, from the east and the country’s narrow interior come clouds that bubble up into a fierce but brief rainstorm; soon sheets of water come pouring down from the mountains and crashing into the wind-whipped Tyrrhenian Sea.

Mountains such as the Serra del Tuono create their own weather; they call up clouds and thunder from seemingly nowhere, as if calling for a drink of water. This is as true of mountains tucked away in the remotest corners of the desert as it is for those in the tropics, for mountains form natural impediments that milk the moisture from sinking, water-laden air masses while routing hot, dry winds upward in thermal columns. Anabatic and katabatic, or upward-flowing and downward-flowing, winds surround most mountains, lending the airspace above the rocks a choppy quality that airline passengers well know, and they play a role in several aspects of continental weather patterns, including the so-called rainshadow effect of the deserts of western North America and the autumn foehn of the southern Alps, a warm dry wind that sometimes melts the first snows of autumn and imperils late-season mountaineers.

Geology and climatology conspire to make mountains important sources of water, releasing their bounty to the surrounding lowlands in regular and usually predictable patterns. As they do, they draw on stores of water that are naturally within the rock—for, just as mountains are laced with veins of ore, so, too, has volcanic action forced veins of underground water up to the highest summits, forming the springs and tarns that give birth to so many of the world’s rivers.

Even in the most forbidding deserts, mountain springs do their work. In the Tassili Mountains, for instance, in the heart of the Sahara, stand ancient groves of well-watered cypress trees and small lakes full of fish, all thanks to the range’s talent for storing water. In the mountains of southeastern California, wind caves deep within the rock shelter groves of palm trees long extinct elsewhere, providing water for dozens of species of plants and animals—including wayward humans. Water even seeps from the great monolith called Uluru, deep within the continent-wide Australian Desert—water that lends its name to the town of Alice Springs.

Another desert town that benefits from the presence the great stone water tanks that mountains afford is Chinle, Arizona, at the mouth of Canyon de Chelly. The little town takes its name from the Navajo words meaning “where the water comes out,” and if you follow that water, the long stream called Chinle Creek, through the deep, whitewalled canyon complex and over the rugged escarpment called the Defiance Uplift, you will eventually find one of its sources in the Tunitcha Mountains, whose name means something like “where much water comes from.” That name fits perfectly, for not only do the often snowclad mountains shed water into abundant creeks feeding into the distant Colorado River, providing two-thirds of the surface water found within the entire Navajo Nation, but the top of 9,512-foot-tall Matthews Peak, the Tunitchas’ highest point, is also dotted with natural springs and waterholes.

In more temperate climates, the mountains are correspondingly generous, which is why many mountain chains there are pockmarked by dams such as the Hetch Hetchy of California and the Three Gorges of China. The stores of water within the ancient rock in such places are augmented by rainfall carried in by seasonal winds, often in quantities many times greater than the rainfall of the neighboring lowlands. If a mountain is well forested or contains adequate natural storage facilities, this rainfall will percolate down to the lowlands at a more or less even pace; where mountain areas have been heavily logged or lack reservoirs and water-draining meadows, there is always the danger that highland storms will lead to lowland flooding, as in the case of the Big Thompson flood of 1976, which saw extraordinarily heavy rainfall—more than 14 inches (36 cm) in six hours—rocketing down a steep, narrow canyon without side drainages. In the end, 139 people died.

Water stored within the rock is only part of the equation, though. By virtue of their weather-transforming abilities, mountains attract precipitation in the form of rain and snow. Although rain is a more economical form of precipitation, the ability of a mountain to store water in frozen form throughout the winter months is an essential for lowland life. In this respect, a mountain is a sort of savings bank whose interest payment comes in the form of a good harvest. “May Kabul be without gold rather than without snow,” goes the Afghani proverb, speaking to the heart of the matter: if the Hindu Kush were suddenly to go snowless, that city would die—as would Denver without the snow of the Rockies, New York without the snow of the Adirondacks, Milan without the snow of the Alps, and so forth.

So far, this has been a snowy year across much of the Northern Hemisphere. Yet snowless years are becoming more common around the world, and scientists point to global warming as the cause. The Rocky Mountains region in particular has suffered from a long drought, while the loss of glacial fields on mountains such as Kilimanjaro and Mont Blanc suggests that a major climatological shift is in play. Even the snowpack on Everest, the highest mountain in the world, has been declining, for which reason scientists in 2004 proposed adding it to the World Heritage Danger list, arguing that the disruption of regional water cycles could prove catastrophic. No one is quite certain how to keep the well from running dry, but the mountains stand ready for the day that the snows fall in abundance again.